Late-season nitrate test helps fine-tune corn fertilization

• The test provides a good assessment of whether the corn crop had the right amount of nitrogen, too much nitrogen, or whether it ran out of gas.
• This information combined with records of nitrogen management can be very useful for making and fine tuning future nitrogen and manure management decisions.

By Douglas Beegle, Penn State University | Sep 02, 2011

The Late-Season Cornstalk Nitrate Test (LSSNT) has been demonstrated to be a reliable end-of-season indicator of crop nitrogen status.

It provides a good assessment of whether the crop had the right amount of nitrogen, too much nitrogen, or whether it ran out of gas.

This information combined with records of nitrogen management can be very useful for making and fine tuning future nitrogen and manure management decisions. See Agronomy Factsheet #70 Late Season Cornstalk Nitrate Test (http://cropsoil.psu.edu/extension/facts/agfact70.pdf) for all of the details.

Sampling

For the test results to be valid the sampling instructions must be followed carefully. Samples for this test should be taken between ¼ milk line and up to 3 weeks after black layer.

An 8-inch long section of corn stalk starting 6 inches above the ground is collected from at least 10 representative plants in a field. After the sample is collected the stalk segments should be cut into shorter pieces to promote drying and then sent to the lab for analysis.

Video demonstrates procedure

Extension educators Jeff Graybill and John Rowehl have produced a short video titled “Using the Late Season Corn Stalk Nitrate Test” (http://cmeg.psu.edu/video/stalk_test/stalk_test.cfm) which demonstrates the proper procedures for taking Late-Season Cornstalk Nitrate Test samples. This video can be accessed at the link above.

Weather

A common question this year is how will the weather we have had affect this test? The general response is that results this year must be interpreted in light of the “unusual” weather this year; wet early with late planting, followed by very dry mid-summer, and now generally wet conditions.

And then there are some more specific things. The wet weather at different times of the season could have resulted in significant loss of nitrogen, which might result in lower stalk nitrate levels.

However, the later planting because of the wet spring and the dry conditions later have resulted in generally lower yields, and thus, nitrogen demand by the plant could result in higher stalk nitrogen levels.

The key thing will be to look at the management and what the test is indicating.

Specific timing, method, and form of nitrogen applied are important factors to evaluate in relation to the LSSNT. For example, with the early wet period the test could help to indicate the losses due to early nitrogen application or the benefits of delayed application depending on the management used.

If nitrogen amendments were used, the LSSNT could help indicate how well these worked under the conditions this year. This would be especially helpful if there were check plots for comparison.

Since the best value from this test is to evaluate nitrogen management over time, this year’s data should probably have an asterisk as you look at your results over time. (Unless this is the new normal?)

Following silage harvest

Research has shown that samples could be taken up to 24 hours following silage harvest. For this to work the corn must be chopped at least 14 inches high where the sample is to be taken so you can still get the correct sample i.e., 8-inch long sections of corn stalk starting 6 inches above the ground.

Some farmers will raise the chopper head occasionally to leave some taller stubble to facilitate sampling later. Don't delay any longer and, in general, be careful if there is any stalk deterioration.

Results

If the results of the test fall between 700 and 2000 ppm nitrate-nitrogen, this indicates the nitrogen management was optimum. Below this range the crop likely ran out of nitrogen and did not achieve full yield potential. Results above this range indicate the crop had more than enough nitrogen which could represent an economic loss from purchasing unnecessary fertilizer nitrogen or wasted manure nitrogen and it could result in increased potential for loss of nitrogen to the environment.

Compare the results with management records to determine the possible reasons for the outcome and use this to make adjustments in future management.